1. Field of the Invention
The present invention relates to a technique of suppressing degradation of image quality due to foreign substances adhering to the surface of an optical low-pass filter or the like in an image capturing apparatus using an image sensor such as a CCD or a CMOS sensor.
2. Description of the Related Art
Recently, many kinds of image capturing apparatuses which generate image signals by using image sensors such as CCDs and record them as data, for example, digital cameras and digital video cameras, have come to be on the market. Digital cameras need not use any photosensitive films, which have been used as recording media, and record images as data on data recording media such as semiconductor memory cards and hard disk drives are used in place of such films. These data recording media allow write and erase operations many times, and hence can save expenses for consumables. That is, such media are very convenient.
In general, a digital camera is equipped with an LCD (Liquid Crystal Display) monitor device capable of displaying captured images as needed and a detachable large-capacity storage device.
Using a digital camera comprising these two devices makes it unnecessary to use any films as recording media which have been used as consumables and allows immediate checking of captured images by displaying them on the LCD monitor device. This makes it possible to erase unsatisfactory image data on the site or recapture the same image as needed. That is, this camera has remarkably improved the photo capturing efficiency as compared with a silver halide camera using films.
Such convenience and technical innovations such as an increase in the number of pixels of an image sensor have widened the application range of digital cameras. Recently, many lens-interchangeable digital cameras such as single-lens reflex cameras have been available.
Foreign substances such as dust and foreign particles (to be simply referred to as foreign substances hereinafter) sometimes adhere to the surfaces of the image sensor protective glass fixed to the image sensor and of an optical filter and the like (to be collectively referred to as image sensor optical system components hereinafter) arranged near the image sensor. When a foreign substance adheres to an image sensor optical system component in this manner, the foreign substance blocks light. In this case, for example, an object image corresponding to that portion cannot be obtained, resulting in degradation of the image quality of the captured image.
Not only a digital camera but also a camera using silver halide films suffers the problem that a foreign substance on a film is reflected in a frame. In a camera using films, however, since a film moves frame by frame, the same foreign substance is rarely reflected in all the frames.
In contrast to this, the image sensor of a digital camera does not move. That is, the camera performs image capturing by using the same image sensor. If, therefore, a foreign substance adheres to an image sensor optical system component, the same foreign substance is reflected in many frames (captured images). A lens-interchangeable digital camera, in particular, suffers a problem that a foreign substance tends to enter the camera when the user interchanges lenses.
The user therefore must always pay attention to the adhesion of a foreign substance to an image sensor optical system component, and spends much effort to check and clean foreign substances. Since an image sensor is placed relatively deep in the camera, in particular, it is not easy to check and clean foreign substances.
A foreign substance readily enters the lens-interchangeable digital camera during lens attaching/detaching operation. In addition, most of lens-interchangeable digital cameras have a focal-plane shutter in front of the image sensor, and a foreign substance easily adheres to an image sensor optical system component.
A foreign substance on such an image sensor usually adheres to the protective glass or the optical filter instead of the surface of the image sensor, and hence the image forming state of the foreign substance varies depending on the aperture value or pupil position of the photographing lens. That is, as the aperture value becomes closer to the full open aperture value, a foreign substance blurs. Even if a small foreign substance adheres to such a component, there is almost no influence on the captured image. In contrast, as the aperture value increases, the foreign substance is clearly focused to influence the captured image.
There is known a method of making a foreign substance less noticeable by using a combination of a normally captured image and an image of only a foreign substance on the image sensor which is obtained by capturing a white wall or the like while the lens is stopped down (see Japanese Patent Laid-Open No. 2004-222231). This method is, however, cumbersome to the user because he/she must always be conscious of the correspondence between an image captured for the detection of a foreign substance and normally captured images to be associated with it.
It is known that as the f-number in normal image capturing decreases (i.e., the aperture value of the lens becomes closer to the full open aperture value), a foreign substance image blurs and becomes less noticeable. Performing analysis processing or interpolation processing while a foreign substance image blurs and becomes less noticeable tends to cause errors. This may lead to deterioration of image quality.